Television transmitting tube



e. A. MORTON El AL. 2.1809 TELEvIsIori TRANSMITTING TUBE 2 Shets-Sheet 1 Filed Jan. 30, 19:55

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TELEVI SIQN TRANSMITTING TUBE Filed Jan. so, 1935 2 Sheets-Sheet 2 'lllllllllll-- o wm K W m m mm m m m A Patented Nov. 21, 1939 TELEVISION TRANSMITTING TUBE tion of Delaware Application January 30, 1935, Serial No. 4,024

1 Claim.

Our invention relates to television transmitting tubes of the type wherein an optical image of an object is translated into an electrical image and the electrical image, in turn, is translated into a 5 train of electrical impulses.

More especially, our invention relates to transmitting tubes of the type wherein an optical image is utilized to produce an electrical charge or potential image which corresponds, point by point, thereto. For the purpose of producing the electrical image, heretofore it has been customary to focus the optical image directly upon a screen constituted by a plurality of minute photosensitive metallic globules or particles supported 15 on the surface of an electrode common thereto, the globules being insulated from each other and from the electrode. In the operation of such a tube, the photo-electrons emitted by the globules cause each of them to acquire a positive charge 20 with respect to the common electrode or signalplate, each charge being proportional to the intensity and color of that elemental portion of the optical image falling upon the emitting globule. These charges are successively neutralized by a cathode ray to produce a train of electrical impulses, in an output device such as a resistor, representing the optical image.

Mosaic screen transmitting tubes, of the type referred to above, have great sensitivity. The

30 limit of sensitivity, however, with known photoelectric materials such as caesium, rubidium, potassium and sodium, seems to have been about reached in tubes wherein the screen is directly exposed to light from the object being tele- 35 Vised.

It is, accordingly, the principal object of our invention to provide a television transmitting tube that shall have considerably more sensitivity than mosaic screen tubes of the direct exposure 40 type heretofore manufactured.

Another object of our invention is to provide a television transmitting tube wherein the phenomenon of secondary emission is made use of for the purpose of intensifying the electrical image.

5 Another object of our invention is to provide a television transmitting tube of the general type under discussion wherein the electrical image is intensified before the scanning operation takes place.

30 A transmitting tube constructed in accordance with our invention includes, within an evacuated envelope, in the order given, a controllable electron source, a, first anode, a second anode, a modified mosaic screen, a reticulated electron-accel- ;5 erating electrode and a planar photosensitive electrode. In the operation of the tube, an optical image is focused upon the planar electrode thus causingvit to emit. photo-electrons. The photo-electrons are focused, as a current image, upon the mosaic screen in any convenient way, 5 such as by a magnetic field. The reticulated electrode, maintained at a high positive potential with respect to the planar electrodes, accelerates the photo-electrons which, after they pass through the meshes of the screen, strike the globules of the mosaic screen with sufficient velocity to cause secondary emission therefrom.

Inasmuch as the current image truly represents the optical image, each elemental portion of it, impinging upon the minute isolated metal elements'of the mosaic screen, causes the emission of secondary electrons proportionally thereto, thus charging each of the said elements to a positive potential which is alsoproportional. Neglecting for a moment the effect of the scanning ray, it will be seen that a charge image is formed on the screen, each elemental portion of which represents the illumination of the corresponding elemental portion of the first photosensitive electrode. 25.

The charge image may be neutralized by a cathode ray which scans the mosaic screen on the side exposed to the primary photo-electrons but this leads to undesired complication in the tube structure. Accordingly, in the preferred 3Q embodiment of our invention under discussion, we cause the cathode ray to scan the opposite side of the screen structure, successively neutralizing discrete charges acquired by the tiny metallic elements thereof, thus producing a train of electrical impulses in an appropriate external output device, such as a resistor, connected to the signal-plate.

The elements of the mosaic are treated to increase their capability of secondary emission. So 40 far, as a matter of experience, we have found that the secondary emission from photosensitized metallic surfaces is in excess of that from untreated surfaces. For this reason we prefer to photosensitize the surfaces of the globules exposed to the photo-electrons, but it is to be clearly understood that other methods of sensitization may be employed.

The novel features which we consider characteristic of our invention are set forth with particularity in the appended claim. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be un- .cl rstood from the following description of a spe- Fig. 5 is a diagrammatic View exemplifying the I manner in which our improved tube is utilized in television transmitters and the like.

Referring now to Fig. 1, a tube constructed according to our invention comprises a glass envelope I, having a neck portion 3 in which is mounted an electron source 5, or gun, and a body portion 1 coaxial therewith wherein are disposed in the order given, a mosaic screen structure 9, an electron-permeable reticulated accelerating electrode H and a planar photosensitive electrode I3. The photosensitive electrode I3 is preferably made of silver and the accelerating electrode ll of nickel, molybdenum, tantalum or any other metal which is readily de-gassed and is not easily oxidized.

As is more clearly shown in Fig. 2, the electron gun includes a cathode thimble I5 on the closed end of which is a layer of electron emissive oxides I1, and which is provided with a heater IS. A cylindrical grid element 2|, provided with an opening 23 coaxial with the end of the cathode thimble and closely adjacent tothe layer of electron emitting oxides carried thereby, surrounds the cathode thimble and a cylindrical first anode 25 is supported in alignment therewith. The opposed ends of the cylinders constituting the grid and first anode, respectively, are each telescopically fitted into an encirclingring 21 of insulating material, such as lavite. They are maintained in fixed position by means of tie rods 29 extending between them, the opposed ends of which are fastened. together by beads 3| of insulating material, such as glass.

It will be noted that the mosaic screen assembly, the reticulated electrode and the planar photosensitive electrode lie in planes spaced apart along the long axis of the tube and normal thereto. This spacing is maintained by a plurality of tie rods 33, 35, 31 and 39, of nickel or the like, from which leads 4|, 43, 45 and 41, respectively, extend exteriorly of the container. The tie rod 33 is conductively connected to the reticulated electrode, the tie rod 35 is connected to the planar photosensitive electrode I3 and has an insulated extension 49 which aids in supporting the reticulated electrode, and the tie rod 31 is conductively connected tothe signalplate portion of the mosaic screen (hereinafter to be described). The tie rod 39 is shown in the drawings as being provided with an external lead, as it actually appeared in an experimental tube, but this rod has no function other than to contribute to the spacing apart and support of the mosaic screen and the photosensitive electrode.

Digressing for the moment, attention is called to Figs. 3 and 4. As is exemplified in Fig. l and as more clearly shown in Fig. 3, the mosaic screen-assembly 9 is constituted by two discs of mica 5|, each having a central opening 53 p enamel.

through which the screen is visible. The mosaic screen per se is clamped between these discs and. is held in position by a metallic ring 55 which rests against one disc and is held in contact with the other disc by a plurality of rivets 51.

The screen per se, as exemplified by Fig. 4, comprises a section of mesh, or finer, wire screen, the wires 59 of which have been so coated with insulating enamel 6| as to leave each of the tiny openings 63 unobstructed. Over each of the openings on one side of the screen is disposed a tiny metallic element or globule 65, preferably of silver, having a surface S'l so sensitized as to be capable of high secondary emission. The globules are fused to the insulating The opposite side of the screen may be provided with a metallic coating 69 which surrounds each of the openings and does not obstruct them. This coating, or barrier-grid, is not absolutely essential. The exact method of manufacture of the mosaic screen forms no part of our present invention but it may be made as described in a copending application, in the name of George A. Morton, filed Jan. 30, 1935, Ser. No. 4,023, and assigned to Radio Corporation of America.

Hereinafter, when the word screen is used, it is to be construed as meaning the structure shown in partial cross-section in Fig. 4, the phrase signal plate is to be understood as meaning the metallic screen per se or both the metallic screen and the metallic coating carried by the insulated screen on the side opposite to the side to which the metallic globules are affixed when such a coating is used. In certain instances, it is desirable to connect the wire screen and the metallic coating together to constitute the signal plate; in other instances separate leads may be brought out from each, and either or both leads utilized.

A metallic electrode H in the form of an interior surface coating is carried by the container intermediate the first anode and the mosaic screen assembly. This is a cathode-ray accelerating and focusing electrode and it also functions to remove secondary electrons emitted from the mosaic screen when bombarded by the cathode ray. This electrode hereinafter will be referred to as the second anode.

In manufacturing a tube such as has been described, it is, of course, impossible to introduce the large electrodes into it through the neck portion. Accordingly, we have found it expedient to first mount the electron gun in the neck portion, which previously has been given a conductive coating, such as platinum, silver, carbon, etc., to provide the second anode, and thereafter to fuse to it the enlarged portion in which the mosaic screen assembly, the reticulated electrode and the planar photosensitive electrode have been previously positioned. To this end the large portion of the tube may be divided intermediate the reticulated electrode and the photosensitive electrode or at any other convenient point.

After the several sections of the tube have been fused together to constitute a complete container, as shown in Fig. 1, the tube is heated and evacuated. After evacuation, oxygen is introduced into the container, at a pressure in the neighborhood of 1 mm. of mercury, for the purpose of oxidizing the surface of the planar electrode and the surface of each of the mosaicglobules opposed thereto. The oxidation of the mosaic globules may be accomplished by causing a glow discharge to take place between them and the reticulated electrode or an exterior, movable electrode. The planar electrode surface may be oxidized by setting up an electrostatic field between it and an exterior terminal of a high potential, high frequency source. Alternatively, both of the electrode surfaces may be oxidized by causing eddy currents to fiow in them, as is customary in de-gassing the electrodes of ordinary thermionic tubes. The oxidation is customarily carried on until the silver surfaces acquire a bluish-green tinge.

Following the oxidation, and after the residual oxygen has been pumped out, a small amount of metallic caesium is distilled into the tube between the screen and the planar electrode from a side tube which is subsequently cut off usually leaving a small projection, clearly shown in the drawings.

The container is next baked for about ten minutes at a temperature of 210 Centigrade, which causes the caesium to combine with the silver oxide, giving rise to surfaces which copiously emit secondary elections when bombarded and which are photo-sensitive. During the baking, excess caesium may be pumped out of the container, since it does not adhere to the container wall or to the reticulated electrode and the tie rods.

It will be noted that the mica discs included in the mosaic screen assembly make close contact with the inner wall of the container, thus preventing the caesium, in large measure, from leaking past and combining with the second anode.

In the operation of our improved television transmitting tube, referring now to Fig. of the drawings, an optical image of an object '53 is focused upon the photosensitive electrode it, which by a suitable source '55, is maintained negative with respect to the second anode ll. The reticulated electrode H, by a second suitable source I1, is maintained positive with respect to the second anode H.

When an optical image is focused upon the photosensitive electrodes, photo-electrons are emitted. These photo-electrons, constituting a current image, are accelerated toward the reticulated electrode 5 l and, passing through its meshes, fall with high velocity onto the exposed surfaces 57 of the minute silver globules 65 of the mosaic screen, causing the emission of secondary electrons therefrom.

In order that the current image shall be focused onto the mosaic screen, we may provide means for setting up a magnetic field symmetrical around the long axis of the tube. Such means may be constituted, as shown in Fig. 5 of the drawings, by a coil l3 surrounding the large portion of the tube intermediate the mosaic screen and the photosensitive electrode, which coil is supplied with direct current from any suitable source 8!. As is now well known to those skilled in the art, if the strength of the magnetic field is properly correlated to the other parameters of the system, electrons emitted from any given elemental area of the photosensitive electrode will be brought to a focus upon a corresponding area of the mosaic screen. Accordingly, the photoelectrons, in bombarding the exposed portions of the caesiated silver globules drive ofi secondary electrons proportional to the optical image. The result is that each small globule acquires a charge in the positive sense with respect to the signal plate constituted by the supporting wire mesh and the metallic coating 69 on the opposite surface thereof, if such coating is present.

Externally of the tube the signal plate may be connected through a resistor 83and a biasing source 85 to the second anode and also to ground. The biasing source maintains the signal plate a few volts negative with respect to the second anode. If the metallic coating is not present on the screen, the biasing source may be omitted. The first anode in the tube is maintained, by an appropriate source 81', from 200 to 300 volts positive with respect to the cathode and the same source, as shown in the drawings, may be utilized to maintain the second anode at a potential in neighborhood of 1000 volts positive with respect to the cathode.

By means of a plurality of deflection coils 89 and Hi, the cathode ray is caused to repeatedly move rapidly over the screen in a horizontal direction and more slowly in a vertical direction in order that it may be caused to successively pass through the openings opposite the small globules and to impinge upon the globules to neutralize the charges thereof and to drive them to equilibrium.

The successive neutralization of the charges produced on the globules by secondary emission gives rise to a train of impulses in the resistor 83 connected between the signal plate and ground. This resistor is included in the input circuit of an amplifier tube 93 having an output device, such as a resistor 95, across which amplified potentials corresponding to the train of impulses appear.

Recapitulating, in a few words the operation of our improved tube is as follows:

An optical image is utilized to cause a current image from a photosensitive electrode, the current image is caused to impinge at high velocity upon a mosaic electrode constituted by a plurality of secondary emitters, insulatingly supported on a common signal plate, to thereby drive off secondary electrons. In this process each globule acquires positive charge proportional to the corresponding elemental section of the current image which it receives and these elemental charges are successively neutralized by a cathode ray which is directed toward and falls upon the globules from the side opposite to the side which receives the current image. Successive neutralization of the charges produces a train of electrical impulses, in an output device, which may be suitably amplified by any appropriate means.

In view of the fact that the globules of the mosaic screen emit secondary electrons greatly in excess of the primary electrons striking them, it will thus be seen that in our improved tube the electrical image is amplified before the scanning operation. The primary electrical image may be amplified up to ten or more times, giving rise to a device which is many more times more sensitive than analogus devices constructed according to prior art. That is to say, our improved transmitting tube provides a signal of greater amplitude than devices heretofore known, for a given light intensity, thus permitting its use in a television transmitter under conditions which render such older devices entirely inoperative.

Although we have described only a single embodiment of our invention we are fully aware of many modifications that will at once be apparent to those skilled in the art to which it pertains. Our invention, therefore, is not to be limited except insofar as is necessitated by prior art and by the spirit of the appended claim.

We claim as our invention:

Apparatus for television and the like including light responsive means for translating an optical image into an electrical current image, means for generating a substantially concentrated beam of electrons, a mosaic electrode positioned between said first named means and said beam generating means, a planar electron permeable mosaic electrode positioned between said first named means and said mosaic electrode, means for applying potential to the mosaic electrode which is positive relative to both the light responsive means and the beam generating means for accelerating the electrons forming the current image in a substantially direct path toward the mosaic whereby the said electrons may be caused to impact one side of the mosaic electrode, means including a secondary emissive layer on said one side of said mosaic electrode for producing secondary electrons in response to bombardment by the electrons of the current image whereby the current image is converted into an intensified electrostatic image, auxiliary means for maintaining the current image in substantially constant spatial focus, means for scanning by the concentrated electron beam that side of the mosaic remote from the secondary electron emisj sive layer to develop therebyla series of picture signals of intensity proportional to the electrostatic charge image existing on the mosaic electrode, and a load circuit responsive to the developed picture signals.

GEORGE A. MORTON. ARTHUR W. VAN CE.v 

